1. Field of the Invention
The present invention relates to an inkjet printhead and, more particularly, to a thermal inkjet printhead in which heat is prevented from accumulating around a heater, thereby improving ink ejection capability.
2. Description of the Related Art
An inkjet printhead is an apparatus that ejects minute ink droplets on desired positions of recording paper in order to print predetermined color images. Inkjet printers are classified into a shuttle type inkjet printer whose printhead is shuttled in a direction perpendicular to the direction of transporting a print medium to print an image and a line printing type inkjet printer having a page-wide array printhead corresponding to the width of a print medium. The latter has been developed for realizing high-speed printing. The array printhead has a plurality of inkjet printheads arranged in a predetermined configuration. In the line printing type inkjet printer, during printing, the array printhead is fixed and a print medium is transported, thereby enabling high-speed printing.
Inkjet printheads are categorized into two types according to the ink droplet ejection mechanism thereof. The first one is a thermal inkjet printhead that ejects ink droplets due to an expansion force of ink bubbles generated by thermal energy. The other one is a piezoelectric inkjet printhead that ejects ink droplets by a pressure applied to ink due to the deformation of a piezoelectric body.
The ink droplet ejection mechanism of the thermal inkjet printhead is as follows. When a current flows through a heater made of a heating resistor, the heater is heated and ink near the heater in an ink chamber is instantaneously heated up to about 300° C. Accordingly, ink bubbles are generated by ink evaporation, and the generated bubbles are expanded to exert a pressure on the ink filled in the ink chamber. Thereafter, an ink droplet is ejected through a nozzle out of the ink chamber.
FIG. 1 is a schematic view of a cross-sectional view of a conventional thermal inkjet printhead. Referring to FIG. 1, the conventional inkjet printhead includes a substrate 10 on which a plurality of material layers are formed, a chamber layer 20 stacked on the substrate 10, and a nozzle layer 30 stacked on the chamber layer 20. An ink chamber 22 filled with ink to be ejected is formed in the chamber layer 20 and a nozzle 32 through which ink is ejected is formed in the nozzle layer 30. In addition, the substrate 10 has an ink feed hole 11 to supply ink to the ink chamber 22.
A typical silicon substrate is used as the substrate 10. An insulating layer 12 for insulation between a heater 13 and the substrate 10 is formed on the substrate 10. The insulating layer 12 is typically made of silicon oxide. Though not illustrated in the drawings, a plurality of CMOS for driving the heater 13 are formed on the substrate 10 and wires for electrically connecting the CMOS and the heater 13 are formed in a plurality of layers inside the insulating layer 12. The heater 13 is formed on the insulating layer 12 to heat the ink of the ink chamber 22 and generate bubble. An electrode 14 is formed on the heater 13 to apply current to the heater 13.
A passivation layer 15 is formed on the heater 13 and the electrode 14 to protect the heater 13 and the electrode 14. The passivation layer 15 is typically made of silicon oxide or silicon nitride. An anti-cavitation layer 16 is formed on the passivation layer 15. The anti-cavitation layer 16 protects the heater 13 from a cavitation force when the bubbles vanish and is typically made of tantalum (Ta).
In the above configuration, the heat produced by the heater 13 and not used to generate ink bubbles must be dissipated toward the substrate 10 through the insulating layer 12 formed under the heater 13. However, as the insulating layer 12 is made of silicon oxide, which has low thermal conductivity, the heat generated by the heater 13 is not dissipated toward the substrate 10 and is accumulated around the heater 13. Meanwhile, since wires are formed in a plurality of layers inside the insulating layer 12, it is difficult to reduce the thickness of the insulating layer 12 so that heat can be dissipated toward the substrate 10. The heat accumulated inside the insulating layer 12 increases the temperature of the ink filled in the ink chamber 22 and thus changes the ink viscosity, and the change of the ink viscosity deteriorates the ejection frequency and speed of the ink.
Recently, as high integration and high speed for printheads are required, line printing type inkjet printers have been actively developed. Such line printing type printers include array printheads with a large number of heaters that generate much heat. Accordingly, when the conventional thermal inkjet printheads are used for array printheads, the ink ejection capability thereof may deteriorate even more.